The invention relates to a television transmitter comprising a vision signal amplifier, a second signal amplifier, a sound and vision combining filter means for generating two frequency modulated sound carrier signals, and means for feeding the frequency modulated sound carrier signals to the sound signal amplifier.
Various proposals have been made for transmitting stereo sound with television pictures or for providing dual channel sound so that a program can be heard in one of two languages selected by the viewer. Such systems are already in operation in the Federal Republic of Germany and trials have taken place elsewhere.
The two sound carrier signals may typically be separated by 250 KHz and this causes problems with the combining filter. This filter normally comprises a notch filter tuned to the sound carrier frequency so that amplitude modulation, caused by the filter due to frequency modulation of the sound carrier, is kept to a minimum. This is a requirement of the Broadcasting Authorities who lay down specifications for the signals to be transmitted. The passband of the notch filter is insufficient to allow both modulated sound carriers to meet the current specifications. This problem has been overcome by modifying the combining filter to provide a broader passband, effectively by stagger-tuning two cascaded filters. However, this is an expensive solution since the filters have to handle high power signals at ultra high frequencies.